This invention relates to image signal processing devices, and more particularly to an image signal processing device in which the data of an original such as a document or a picture are optically read and converted into electrical signals, which are binary-coded with reference to a threshold value.
Heretofore, in binary-coding the electrical signals obtained by optically reading the data on originals such as documents and/or pictures into white and black level signals, a binary coding system has generally been employed in which a fixed threshold value is provided; the electrical signals being compared with the threshold value.
FIG. 1 shows the relationships between such signals and the threshold value of the binary coding system. In FIG. 1, reference character A designates a white level; B, a black level; and C, a fixed threshold value.
The system is effective for a signal (a) read from a clear original which is low in background density. However, the system is still disadvantageous in that, for a signal (b) read from an original which is high in background density or a signal (c) read from an original where the document and/or picture is low in density, it is impossible to reproduce the image with high fidelity.
A system is also known in the art in which the threshold value may be varied for binary-coding the electrical signals. However, this system is also disadvanageous in that, since the threshold value must be manually indicated, the operation is considerably intricate.